The invention concerns a process used to trigger a passive occupant protection system for motor vehicles where at least one electric ignition agent for a safety device protecting one person in the event of a hazardous impact, in particular an airbag or a belt tensioner, is provided with ignition energy by means of a trigger current which, if a function defect occurs in the on-board electrical system of a motor vehicle, is generated as a discharge current of an autarchic capacitor, and where this autarchic capacitor provides the trigger currents for multiple ignition agents simultaneously. In addition the invention also concerns a circuit for implementing this process.
A process of this type is known from DE 43 19 001 A1.
With regard to occupant protection systems for motor vehicles such as for example airbag systems or belt tensioners, a very high degree of system reliability is required as they are particularly critical to safety.
Airbag systems usually feature one trigger circuit for each airbag, complete with an ignition agent activating the airbag, a so-called squib, which is located in series to a controllable switch or--for reasons of safety--in series between two controllable switches. In the event of a hazardous impact, a control unit interacting with accelerometers will activate the ignition agents by closing the controllable switches contained in the trigger circuit by means of a trigger signal.
Furthermore, it is known that, if the on-board electrical system of a motor vehicle fails or if a function defect occurs in the electrical system, a safety capacitor (autarchic capacitor) is to be provided which ensures that even in such cases electrical energy is supplied to the ignition agent. Such an autarchic capacitor is known from DE 39 13 628 A1, which supplies not only the ignition agent with the necessary trigger current but also simultaneously the trigger circuit for the ignition agent. Finally, this capacitor will be continuously checked with regard to its charge state in order to ensure its functional reliability.
In addition, a triggering process for an igniter in an airbag is known from DE 44 09 019 by means of which two capacitors are charged up from the on-board electrical system of the motor vehicle. These two capacitors are connected in series to the ignition agents such that, in the event of an impact, sufficient energy will be supplied to the ignition agent in the shortest possible time by means of a closed switch.
Finally, it is known from DE 43 19 001 already mentioned above that, instead of using simultaneously a single autarchic capacitor for multiple ignition agents within an occupant protection system, each ignition agent is to be provided with its own respective autarchic capacitor. As a disadvantage of the simultaneous use of a single autarchic capacitor for multiple ignition agents, this publication states that problems occur inasmuch as the resistance values of the ignition agents are not balanced and that in the event of a short-circuit occurring on one of the ignition agents the occupant protection system cannot be triggered.
The current standard equipment of a motor vehicle comprising two airbags and two belt tensioners therefore requires four power amplifier stages and four autarchic capacitors for energy storage. However, as side airbags are increasingly included in the standard safety equipment of a motor vehicle the number of power amplifier stages and autarchic capacitors would be augmented to six. This leads to a large number of external components which would render such a safety system significantly more expensive.
A further disadvantage is that the ignition agent, i.e. the squib, of the airbag is offered far more energy than is actually required for a safe ignition of the same; there is no justification for this, not even on the grounds of operational safety. For example, the resistance of a squib can adopt a value between 1.6 and 6.6 ohms so that the energy stored in an autarchic capacitor is to be designed for the maximum resistance of a squib. The ignition energy for such a squib, for instance, corresponds to a current of 2 A for a period of 6 ms. If during this time a capacitor with a capacity C=2200 .mu.F is charged with a charge voltage of 27 volts, a maximum initial current of 16.8 A will flow, provided the resistance value of the squib is between 1.6 and 6.6 ohms. This leads to a large chip surface area of the power amplifier stage transistors required for the trigger circuit, as their surface area sizes are determined in proportion to the initial current.